Solid electrolyte ionic or mixed conductor ion transport membranes have been employed to extract oxygen from gases at temperatures within the range of about 500.degree. C. to about 1200.degree. C. The optimum operating temperature for gas transport is dependent on the membrane itself, particularly the material from which it is constructed. Ionic conductivity is also a function of operating temperature, and increases as the operating temperature increases. At operating temperatures less than about 500.degree. C., in addition to the lower ionic conductivity of ion transport membranes, surface kinetic limitations on the membrane may also constrain oxygen flux, that is, the quantity of oxygen per unit area per unit time.
Operating temperatures for ion transport membranes greater than about 1200.degree. C. are also undesirable because material and construction limitations (such as sealing, manifolding and thermal stress) are exacerbated at higher temperatures.
One of the most attractive features of the oxygen ion transport membrane system is the membrane's infinite selectivity for oxygen transport and the fact that this oxygen transport is driven by the ratio of oxygen activities on the opposite sides of the membrane. Thus high oxygen fluxes are possible with a reaction occurring on the anode-side. Also, it is possible to transport oxygen from a low pressure oxygen-containing stream to a high pressure reacting environment.
At elevated temperatures, oxygen-ion transport materials contain mobile oxygen-ion vacancies that provide conduction sites for selective transport of oxygen ions through the material. The transport is driven by the difference in partial pressure across the membrane, as oxygen ions flow from the side with higher partial pressure of oxygen to that with lower partial pressure of oxygen. Ionization of oxygen molecules to oxygen ions takes place on the "cathode-side" of the membrane, and the oxygen ions are then transported across the ion transport membrane. The oxygen ions deionize on the "anode-side" across the membrane to re-form oxygen molecules. For materials that exhibit only ionic conductivity, external electrodes may be placed on the surfaces of the electrolyte and the electronic current is carried in an external circuit. In "mixed conducting" materials, electrons are transported to the cathode internally, thus completing the circuit and obviating the need for external electrodes. Dual phase conductors, in which an oxygen ion conductor is mixed with an electronic conductor, are one type of mixed conductor.
Partial oxidation reactions ("POx") and/or steam reforming reactions involving carbonaceous feedstocks are common methods for producing synthesis gas. Synthesis gas and its major components, carbon monoxide and hydrogen, are valuable industrial gases and important precursors for production of chemicals including ammonia, alcohols (including methanol and higher carbon alcohols), synthesis fuels, acetic acid, aldehydes, ethers, and others Feedstocks including natural gas, coal, naphtha, and fuel oils are commonly used to produce synthesis gas by partial oxidation or steam reforming reactions These reactions may be represented as follows:
C.sub.m H.sub.n +m/2 O.sub.2 =m CO+n/2 H.sub.2 POx, exothermic PA1 C.sub.m H.sub.n +m H.sub.2 O=CO+(m+n/2) H.sub.2 SR, endothermic,
where C.sub.m H.sub.n is a hydrocarbon feedstock.
To improve the rates of reactions and selectivity of certain products, an external catalyst in the form of a fixed or fluidized bed, or a plurality of catalyst tubes, may be used. Individual synthesis gas components, notably hydrogen and carbon monoxide, can be obtained using a number of conventional gas separation methods known in the art such as those based on pressure swing adsorption, temperature swing adsorption, polymeric membranes, and cryogenic distillation Water-gas shift reaction may be carried out to increase the yield of hydrogen by converting the CO in the synthesis gas to H.sub.2 and CO.sub.2 by reaction with steam (CO+H.sub.2 O=CO.sub.2 +H.sub.2).
Conventional partial oxidation processes frequently use oxygen molecules produced by traditional gas separation processes that typically operate at temperatures below 100.degree. C. Since the partial oxidation reaction itself typically requires a high temperature of operation at over 800.degree. C., integration between partial oxidation reaction and traditional oxygen separation has not been realized previously. As a result, conventional partial oxidation reaction has often been characterized by low feedstock conversion, low hydrogen to carbon monoxide ratio, and low hydrogen and carbon monoxide selectivity. Additionally, the external oxygen supply typically required in a partial oxidation reaction adds significantly to capital and operating costs, which may amount to as much as 40% of the total synthesis gas production cost. Moreover, inefficiencies are introduced as the high amount of carbon monoxide gas produced in the partial oxidation reaction product requires a two stage shift conversion when only hydrogen is required as the final product. Shift conversion also adds to the process cost.
The steam reforming reactions are also used for synthesis gas production. Since the steam reforming process produces more hydrogen per mole of organic fuel than the partial oxidation reaction, this process is more advantageous for the production of hydrogen and mixtures with a high H.sub.2 /CO ratio (i.e., a ratio of greater than 2). However, steam reforming is an endothermic reaction requiring a significant amount of thermal energy, and accordingly, is a less attractive method for synthesis gas production when the H.sub.2 /CO ratios are below 2.
In the past, development in the oxygen ion transport membrane system area have included the combination of the membrane system in conjunction with gas turbines. U.S. Pat. Nos. 5,516,359, 5,562,754, 5,565,017 and EPO Patent No. 0,658,366 disclose the production of oxygen in a process that is integrated with a gas turbine system. Commonly assigned U.S. patent application Ser. No. 08/490,362, pending entitled "Method for Producing Oxygen and Generating Power Using a Solid Electrolyte Membrane Integrated with a Gas Turbine" is also directed to oxygen production using ion transport membrane system integrated with gas turbine, and is incorporated herein by reference.
Oxygen-ion transport membrane materials useful for synthesis gas production have been disclosed by U. Balachandran et al., in "Fabrication and Characterization of Dense Ceramic Membranes for Partial Oxidation of Methane", Proc. of Coal Liquefaction and Gas Conversion Contractors' Review Conference, Pittsburgh, Pa. (Aug. 29-31, 1995) and "Dense Ceramic Membranes for Converting Methane to Syngas", submitted to the First International Conference on Ceramic Membranes, 188th meeting to the Electrochemical Society, Inc., Chicago, Ill. (Oct. 8-13, 1995). U.S. Pat. No. 5,306,411 (Mazanec et al.) discloses a process that integrates oxygen separation with partial oxidation (for synthesis gas production) or oxidative coupling of methane.
Despite the emerging technological advances involving ion transport membrane systems, the present inventors are not aware of any disclosure of the practical integration of ion transport membrane systems based on the production of synthesis gas and a hydrogen separation system using solid electrolyte ion transport membrane, and further the separation thereof in a single unit.